An ion exchange membrane is one having a cation exchange group in itself to allow selective permeation of ions and is used in the field of electrodialysis, diffusion dialysis, fuel cells, or the like. Particularly, in the field of fuel cells, an ion exchange membrane is used widely for polymer electrolyte membrane fuel cells (PEMFC), direct methanol fuel cells (DMFC), redox flow batteries (RFB), or the like. An ion exchange membrane provided with lower electrical resistance, higher selective ion permeability, higher chemical stability and higher mechanical strength has higher utility.
Most of commercially available ion exchange membranes include a fluoropolymer having a cation exchange group introduced thereto, and typical examples of such cation exchange membranes include Nafion available from DuPont, Dow membranes available from Dow Chemicals, Aciplex-S membranes available from Asahi Chemicals, and Flemion membranes available from Asahi glass. Particularly, Nafion™ includes a sulfonate group introduced to a polytetrafluoroethylene backbone, and has an ion conductivity of 0.1 S/cm under its saturated moisture content, high mechanical strength and chemical resistance. Thus, it has been used widely as an electrolyte separator in a vanadium redox flow battery. However, Nafion is expensive, requires a complicated fluorine substitution process, and has low ion selectivity. Therefore, active studies have been conducted to develop a polymer capable of substituting for Nafion.
The results of studies that have been conducted to date include ion exchange membranes developed by using non-fluoropolymers and polymers partially substituted with fluorine. Typical examples of such ion exchange membranes include those using polymers based on sulfonated poly(phenylene oxide), poly(phenylene sulfide), polysulfone, poly(para-phenylene), polyetherether ketone, polyimide, or the like (see J. Membr. Sci., 197 (2002) 231; J. Membr. Sci., 185 (2001) 73; J. Electrochem. Soc., 151 (21) (2004) A2150; Micromol. Symp., 175 (2001) 387; J. Membr. Sci., 281 (2006) 121, or the like). For example, Korean Laid-Open Patent No. 2007-0098323 (2007 Oct. 5) discloses a composite electrolyte membrane, including: a first polymer electrolyte layer containing a first non-fluorinated or partially fluorinated sulfonated polymer electrolyte; a non-fluorinated or partially fluorinated microporous polymer substrate disposed on the first polymer electrolyte layer, wherein the pores of the microporous polymer substrate are impregnated with a second non-fluorinated or partially fluorinated polymer electrolyte in such a manner that the first polymer electrolyte is entangled with the second polymer electrolyte on the interface; and a third polymer electrolyte layer disposed on the microporous polymer substrate impregnated with the second polymer electrolyte and containing a third non-fluorinated or partially fluorinated sulfonated polymer electrolyte in such a manner that the second polymer electrolyte is entangled with the third polymer electrolyte on the interface. In addition, Korean Laid-Open Patent No. 2009-0044315 (2009 May 7) discloses a polymer composite electrolyte membrane for a fuel cell, including: a polytetrafluoroethylene microporous membrane impregnated with a non-fluorinated sulfonated polymer electrolyte; and non-fluorinated sulfonated polymer electrolyte layers provided on both surfaces of the polytetrafluoroethylene microporous membrane impregnated with a non-fluorinated sulfonated polymer electrolyte. However, an ion exchange membrane using a sulfonated polymer inevitably undergoes degradation of ion conductivity when it is sulfonated to a concentration higher than its critical concentration, and also undergoes degradation of mechanical properties upon hydration. Thus, such an ion exchange membrane using a sulfonated polymer cannot be used for a long time. In addition, it shows higher electrical resistance as compared to a commercially available membrane, such as Nafion, and thus cannot have a desired level of ion conductivity.
Meanwhile, some studies have been conducted to apply carbon nanotubes (CNT) having a high aspect ratio (300-1000) and excellent mechanical strength (150-180 GPa) to an ion exchange membrane in order to solve the problem of degradation of mechanical properties occurring when an ion exchange membrane is manufactured by using a non-fluorinated polymer. For example, Korean Laid-Open Patent No. 2013-0040022 (2013 Apr. 23) discloses a polymer electrolyte membrane including an ion conductive polymer having an ion conductive functional group, carbon nanotubes having an ion conductive functional group, and a cross-linking agent, a method for manufacturing the same and a fuel cell including the same.
Under these circumstances, the present inventors have conducted many studies to develop an ion exchange membrane having excellent ion conductivity, chemical stability and mechanical strength while also providing high ion selectivity. As a result, the present inventors have found that it is possible to obtain an ion exchange membrane having an ion conductivity and ion selectivity equal to or higher than the ion conductivity and ion selectivity of a commercially available product, when a triblock copolymer is prepared by using a non-fluorinated polymer with a controlled sulfonation degree and cross-linking degree. The present inventors have also found that an ion exchange membrane can resist extra pressure generated by the flow of a solution in a flow battery, when the molecular weight of the ion exchange membrane is increased to improve its mechanical strength. The present invention is based on these findings.